The Stoichiometry Of Leaf Litter-Soil-Enzyme In A Typical Broadleaf Forest Invaded By Moso Bamboo

Vegetation succession alters the ecological stoichiometreristics of carbon(C),nitrogen(N),phosphorus(P)and other elements in the plant-litter-soil system and biochemical cycling,which has important impacts on the forest soil C storage,stability and even global climate change.Soil microorganisms regulate the activity of extracellular enzymes and the decomposition of organic matter according to the availability of environmental nutrients and their own resource requirements,which are the core links that drive the nutrient cycle.As an important forest resource in southern China,Moso bamboo(Phyllostachys edulis)has strong asexual reproduction ability,and can easily spread to neighboring forests through its vigorous rhizome system,then gradually replace the native forests.It has been proved that Moso bamboo invasion significantly changed the vegetation structure,N and P distribution patterns and soil microbiological processes in subtropical evergreen broad-leaved forests.However,how bamboo invasion affects the ecological stoichiometry characteristics of litter-soil-enzymes and the mechanism of regulating microbial activity remain largely unknown.Therefore,this study investigated the quantity of litter input and quality,soil C,N,P and fraction contents,the stoichiometric characteristics of extracellular enzymes,phospholipid fatty acid(PLFA),basal respiration and substrate-induced respiration in three typical forests(a pure Moso bamboo forest(formed by the invasion of evergreen broad-leaved forest),a Moso bamboo-broad-leaved mixed forest,and an evergreen broad-leaved forest).The aim of this study was to reveal the ecological stoichiometric characteristics of litter-soil-extracellular enzymes in the broad-leaved forest habitats invaded by Moso bamboo,and their relationships with microbial nutrient limitation and activity.Our findings can facilitate to understand the status of resource limitation and potential risks of soil C sequestration during Moso bamboo invasion of broad-leaved forests.The main findings are as follows:(1)Forest survey combined with litter input monitoring and quality analysis showed that,bamboo invasion of broad-leaved forest significantly changed the vegetation diversity,aboveground biomass,fine root biomass,and the input amount and quality of litter.Compared with the broad-leaved forest,the aboveground biomass,root biomass and aboveground biomass per unit area in bamboo forest and mixed forest decreased by 87%,90%,51%and 45%,36%,8%on average,respectively,while the tree density and<2 mm fine root biomass increased significantly(P<0.05)by284%,274%and 69%,241%.Bamboo invasion significantly reduced the standing biomass of litter,annual litter production,litter C content,litter C:P and N:P by 59%,49%,9%,29%,19%.Bamboo invasion did not affect the N content of litter,but significantly increased P of litter by 14%,and improved the litter quality.(2)Bamboo invasion significantly changed the contents and stoichiometry of soil C,N and P.Compared with the broad-leaved forests,bamboo invasion reduced soil organic C,total P,C:N,C:P,soluble organic C,and available P by 52%,28%,45%,33%,25%and 41%,respectively,whereas increased the ratio of soluble organic N to available P by 78%(P<0.05).Compared with the broad-leaved forests,soil labile C fraction of I(LPI-C)and II(LPII-C)were reduced by 24%and16%,respectively,and their proportions in SOC were significant(P<0.05)increased by 60%and86%;on the contrary,the recalcitrant C fraction(RP-C)and its proportion in SOC were significantly reduced by 68%and 34%,indicating that bamboo invasion promoted the conversion of recalcitrant C fraction to labile C fraction.(3)Compared with the broad-leaved forests,the concentrations of soil MBC and MBN,and the ratios of MBC:MBN,MBC:MBP and MBN:MBP were significantly(P<0.05)greater by 109%,45%,43%,124%,50%and 52%,46%,4%,72%,71%in the soil of bamboo forest and mixed forest,respectively.The invasion of Moso bamboo significantly reduced the ratio of soil-microbial chemicometric imbalance,which makes the C:N imbalance ratio for the total and labile resource(C:N imbalance)reduced by 16-62%and 55-73%respectively,and the C:P imbalance ratio(C:P imbalance)is reduced by 5-44%and 49-52%respectively(P<0.05).The C:N imbalances for both total and labile pools had significantly positive relationships with annual litter production,litter C concentration and C:P and N:P ratios,SOC,TP,soil C:N,and DOC.(4)Bamboo invasion significantly changed soil enzyme activity,microbial nutrient limitation and C use efficiency.Compared to broadleaf forest,the activities of L-leucine aminopeptidase,N-acetylglucosaminidase,and acid phosphatase(PHOS)significantly decreased by 40%and 50%,33%and 19%,54%and 35%,respectively,under bamboo forest and mixed forest,while there was no change in?-glucosidase(BG)activity(P<0.05).Ecological enzyme stoichiometric analysis showed that,compared with broad forest,the bamboo forest had a significantly higher ratio of BG:PHOS,TER_C:N and C use efficiency,indicating that bamboo invasion mitigated microbial N and P limitation and intensified their C limitation.(5)Bamoo invasion significantly(P<0.05)decreased the total PLFAs content(by 29%),fungi(by 51%),actinomycetes(45%),and the fungi/bacteria ratio(F:B)(by 38%).Bamoo invasion did not affect soil basal respiration,but significantly changed the substrate-induced respiration rate.The bamboo forest and mixed forest soil had a higher utilization rate of carboxylic acid substrates,but a lower utilization rate of amino acids and carbohydrates,and decreased C/N utilization rate ratio(SIR C:N)and microbial metabolic quotient(q CO₂).Structural equation model(SEM)showed that bamboo invasion affected the SIR C:N mainly through affecting the C:N imbalance between soil and microorganisms,the F:B ratio and the TER_C:N.In conclusion,this study demonstrated that Moso bamboo invasion of broad-leaved forest significantly affected the ecological stoichiometric characteristics and balance of litter-soil-microbes,and shifted the structure and metabolic activity of soil microbial communities.Bamboo invasion increased the leaf letter quality,mitigated microbial N and P limitation and aggravated C limitation,which may promote soil organic matter decomposition of broad-leaved forest and nutrients release,subsequently providing a positive feedback for their invasion,but may potentially accelerate the loss of soil C pool.